Nature employs different types of cells to perform routine chemical, reactions in a living organism. By virtue of nano-size compartmentalization and reactant selectivity, a cell can perform cascades of complex reactions with extreme precision and spatial control. A man-made or artificial reaction system which attempts to mimic the proficiency of biological reactors in cells are referred to as nanoreactors. A nanoreactor is typically a compartment that is smaller than one micron in size and that encloses an environment where a reaction may take place in a controlled and well defined manner.
Nanoreactors have been used to develop nanomaterials such as nanoparticles (i.e., metals, metal oxides, metal alloys, metal coated metal oxide, metal oxide coated metal), ceramic materials, and quantum dots. In comparison to their bulk or larger counterparts, nanomaterials exhibit different optical, magnetic, electrical, physical (i.e., mechanical hardness, thermal stability, and chemical passivity), and catalytic properties which may provide different uses of such nanomaterials.
By using principles of self-assembly, various types of nanoreactors have been developed from synthetic and biological building blocks. Examples of such nanoreactors include emulsions, microemulsions. micelles, gels, protein cages, and viruses. The microemulsions (μE) are an efficient system and are clear, thermodynamically stable, colloidal nanodisperions of water in oil or oil in water. The dispersed phase is stabilized within micelles formed by self-assembly of surfactants. Due to Brownian motion, the micelles frequently collide and transiently fuse leading to an exchange of the components within the interior of the micelle. Such dynamic properties facilitate the use of the micelles as confined reaction media and, thus, their utility as nanoreactors.
Advantages of using microemulsions as nanoreactors include: accelerating the rate of reaction up to 100 fold; imparting a cage-like effect which provides good control over particle size which produces particle/nanomaterials with high homogeneity and monodispersity; the surfactant film on the micelles stabilizes the particles and prevents the particles from agglomerating; and the ease of manipulating the properties of the microemulsions enables the fine-tuning of the size and morphology of the nanomaterials.
Water in oil microemulsions have been widely used to produce nanoparticles. However, most of the microemulsions use surfactants that are not biodegradable (i.e., Aerosol OT. Triton X-100, and polyvinylpyrrolidone) and use organic solvents that are hazardous and petroleum based (i.e., iso-octane, heptane, and 1-butanol).
Nanofluids are a class of colloidal systems developed by uniformly dispersing nanomaterials (i.e., nanoparticles, nanofibers, nanotubes, nanowires, nanorods, nanosheets, or nanodroplets) in base fluids. As compared to the base fluids themselves, the nanofluids have different properties such as enhanced thermal conductivity, thermal diffusivity, thermal viscosity, and hear transfer coefficients. Due to the improved thermophysical properties, the nanofluids may be used for applications such as heat transfer, mass transfer, energy storage, tribological uses, and biomedical uses. Nanofluids are categorized as water-based or oil-based. The water-based fluids may be exploited for heat transfer and the oil-based nanofluids may be used in lubricant applications. However, dispersing nanoparticles into oils to make such nanofluids is a challenge and nanofluids are not very stable since nanoparticles often aggregate and precipitate after a few days.
Pure phospholipids such as phosphatidylcholine have been used as a biobased surfactant to produce nanoreactors such as vesicles and liquid crystals. However, lecithin, which is a complex mixture including phospholipids, has not been used to develop nanoreactors. Pure phospholipids are about ten times more expensive than lecithin.